Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Saturn V
Date: Fri, 20 Jun 1997 04:01:31 GMT
In article <33a9684d.8216065@news.zippo.com>,
Tom Carman <71777.517@compuserve.com> wrote:
>>Were the engines intentionally run hydrocarbon rich?
>
>Yes. Rocket engines usually run fuel-rich because it reduces the
>molecular weight of the exhaust, increasing specific impulse.
Actually, the result is right but the proposed mechanism is wrong. (This
is what I meant in my earlier posting when I said that even the textbooks
get the details wrong.) In a chemical rocket, where the reaction mass and
energy source are one and the same, trying to lower the molecular weight
by adding an excess of one propellant also reduces the flame temperature,
and if you actually *do the math*, it's always a net loss. The well-known
sqrt(temperature/molec_wt) factor turns out to be essentially the energy
release per kilogram of exhaust, which will *never* go up if you add extra
propellant which doesn't react.
So why do they run fuel-rich? Well, partly there are some simplifying
assumptions in that math which aren't strictly correct. More important,
though, is that the textbooks look at the wrong part of the equation.
They skip over the nozzle efficiency, which is *not* independent of the
gas composition. In particular, the subexpression (gamma-1)/gamma, which
appears as an *exponent* in the nozzle efficiency, is a strong function of
gas composition, and to a first approximation, it's inversely proportional
to the number of atoms per molecule. So an excess of fuel, meaning that
some of the fuel ends up as CO or H2 rather than CO2 and H2O, can make a
big difference to nozzle efficiency, and that can more than make up for
the reduced energy release.
(If you insist on seeing this in a book rather than doing the math
yourself :-), John Clark's "Ignition!" gets it right -- perhaps because
Clark was a propellant chemist rather than an engine designer, and had
to know *why* running fuel-rich was good, rather than just taking the
computer programs' word for it.)
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu

Newsgroups: sci.space.history
Subject: Re: Fuel-Rich rockets (was Re: Saturn V)
From: Henry Spencer <henry@zoo.toronto.edu>
Date: Wed, 25 Jun 1997 12:12:58 GMT
In article <5op839$fqk@nntp.seflin.org>,
James Wentworth <d005794c@dc.seflin.org> wrote:
>I recall reading that the Space Shuttle Main Engine (SSME) shutdown
>sequence is arranged so that the last propellant that passes through the
>engine is hydrogen, for the same reason...
Note, though, that it's not uncommon, even in the West, to *start* engines
oxidizer-rich. Startup and shutdown sequences are somewhat of a black
art, and the problems are not what you'd call well-understood. Fuel-rich
shutdown is indeed safest, not so much because it's vitally necessary to
always run fuel-rich, as because flow patterns are poorly controlled
during shutdown and hot gases may contact the walls at times.
>I believe the SSME mix ratio is
>fuel rich as well (that could explain the conical blue ?flames we see
>below the SSME nozzles).
The conical flames are shock-wave patterns, I believe. Yes, the SSMEs run
fuel-rich -- all hydrogen engines are run very fuel-rich, because that
improves performance considerably. Having a fair bit of unreacted
hydrogen in the exhaust turns out to be good for performance in several
ways, and hydrogen is so light that the mass penalty for this is small.
Ideally, hydrogen engines would run at about 4:1, with half the hydrogen
unburned; in practice, because hydrogen is so bulky, considerations of
tank mass usually force the engine people to compromise on about 6:1.
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu

Newsgroups: sci.space.history
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Fuel-Rich rockets (was Re: Saturn V)
Date: Thu, 26 Jun 1997 14:53:52 GMT
In article <cbingmanECD0MH.128@netcom.com>,
Craig Bingman <cbingman@netcom.com> wrote:
>>Ideally, hydrogen engines would run at about 4:1, with half the hydrogen
>>unburned; in practice, because hydrogen is so bulky, considerations of
>>tank mass usually force the engine people to compromise on about 6:1.
>
>Do you mean 3:1?
No, those numbers are right. The ratio is usually quoted as O:F, not F:O,
perhaps because in many of the usual combinations the oxidizer outweighs
the fuel and this gives convenient numbers.
8:1 would be the ratio for complete combustion of LOX/LH2. Roughly 4:1
is optimal for rocket performance, depending a bit on design details.
Early hydrogen systems often ran at around 5:1, modern practice is 6:1.
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu

Newsgroups: sci.space.history
Subject: Re: Fuel-Rich rockets (was Re: Saturn V)
From: Henry Spencer <henry@zoo.toronto.edu>
Date: Sat, 28 Jun 1997 05:24:31 GMT
In article <cbingmanECGpAG.9HE@netcom.com>,
Craig Bingman <cbingman@netcom.com> wrote:
>OK. Well, it makes sense. Why would rocket people work with moles
>rather than a mass or weight scale? Only a dumbass chemist like me would do
>that. I should have known better...
Took a bit of adjusting for me too -- I started out to be a chemist, a
longish time ago. The *really* annoying part is that when they write R,
you never know whether that's the universal gas constant as it should be,
or the u.g.c. divided by the molecular weight (which lets them write
PV=MRT instead of PV=nRT, so they don't have to teach engineers what a
mole is :-)). Compared to that, it's hardly even a nuisance that a lot of
their equations still have mysterious factors of g so that they can avoid
having to figure out the difference between weight and mass...
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu

Newsgroups: sci.space.history
Subject: Re: Fuel-Rich rockets (was Re: Saturn V)
From: Henry Spencer <henry@zoo.toronto.edu>
Date: Wed, 25 Jun 1997 11:59:45 GMT
In article <19970624171501.NAA13694@ladder02.news.aol.com>,
PeteAlway <petealway@aol.com> wrote:
>I remember reading in Goddard's notes that when flow problems produced an
>oxygen-rich mix, he'd get burn-throughs of chambers or nozzles. It seems
>the advantage of a fuel rich mix was that the combustion chamber would be
>a reducing, rather than oxidizing, environment for the metal inside the
>rocket engine.
This is a relatively minor concern for modern engine designs. It is
usually necessary, in practice, to design the injector so there is a cool
film of unreacted propellant flowing along the walls. (I'd guess that
Goddard never reached the sort of performance levels where this is
mandatory.) Although it is normal to use fuel for this, and there is
considerable superstition in the West about a fuel-rich mix being
necessary, in fact an oxidizer film works just as well.
When you hear about, e.g., an SSME being damaged on the test stand when
its mix went oxidizer-rich, often what that *really* means is that its
wall-cooling film flow got disrupted.
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu

Newsgroups: sci.space.history
Subject: Re: Fuel-Rich rockets (was Re: Saturn V)
From: Henry Spencer <henry@zoo.toronto.edu>
Date: Fri, 27 Jun 1997 15:24:33 GMT
In article <19970627011700.VAA26390@ladder02.news.aol.com>,
PeteAlway <petealway@aol.com> wrote:
>Of course I got my impressions of the causes of failures from reading
>Goddard, and a modern rocket engineer might have a different take on them.
Yes, chamber cooling is a complicated subject, and one shouldn't take the
tentative opinions of early workers too seriously. This business is
plagued by things being established as gospel too early; for example, the
wonder-fuel mythology surrounding liquid hydrogen dates from the 1940s,
and has only recently been seriously re-examined and found wanting.
Actually, I can think of a subtle reason why oxidizer-film cooling might
*not* have worked for Goddard, had he tried it. Given that he wasn't
doing regenerative cooling, the insulating value of the soot layer that
forms on the walls of hydrocarbon-fueled engines was probably important.
(It can be quite significant even in regeneratively-cooled engines.) An
oxidizer-rich wall film would have prevented soot formation.
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu

Newsgroups: sci.space.history
Subject: Re: Saturn V
From: Henry Spencer <henry@zoo.toronto.edu>
Date: Fri, 27 Jun 1997 00:43:49 GMT
In article <cbingmanECDGn2.941@netcom.com>,
Craig Bingman <cbingman@netcom.com> wrote:
>>They skip over the nozzle efficiency, which is *not* independent of the
>>gas composition...
>
>So, how does this work out for the aerospike engines? They seem to have
>variable nozzle geometry. How does that affect the nozzle efficiency term?
The nozzle efficiency is 1 - (Pe/Pc)^((gamma-1)/gamma). (gamma-1)/gamma
strongly depends on gas composition -- it can also be written R/Cp, where
R is the universal gas constant and Cp is the specific heat at constant
pressure (expressed in the right units!) -- and slightly depends on
various things like temperature. Pe/Pc is exit pressure over chamber
pressure. So for maximum efficiency, you want that second term as small
as possible, meaning very large pressure drop or high R/Cp or both.
The strength of the aerospike is that it automatically adjusts its exit
pressure to match the outside pressure, which gives you the best pressure
drop possible at any given altitude. Its large base area also gives a
rather large pressure drop at high altitude, which is nice because you
do most of your accelerating there and making a conventional nozzle that
big tends to make it heavy.
Good pressure drop doesn't eliminate the need for high R/Cp, but it makes
it a bit less critical. Similarly, a really good R/Cp would reduce the
incentive for pressure drop somewhat.
--
Committees do harm merely by existing. | Henry Spencer
-- Freeman Dyson | henry@zoo.toronto.edu